This invention relates to tennis rackets, and more particularly the manner in which the strings are strung in an ordinary tennis racket.
Nearly all tennis rackets now sold and used are strung in a regular `basket weave` pattern. The longitudinal direction is usually strung first then the cross strings are woven alternately under and over the longitudinal strings. This stringing method entraps each string so that there is very little movement when side forces are imparted to the strings, such as when the ball is struck to impart a spin to the ball. The restoring forces created by the weave pattern try to maintain the regular spacing between the strings.
This common stringing method is fine if the ball is struck on the center of the striking face or slightly below center which is usually the percussion center, frequently referred to as the sweet spot of most tennis rackets. Away from the center up or down or on either side, the strings are shorter between supports, consequently, the resilience decreases, effective spring constant increase. The resilience further decreases because the ball is struck closer to the frame which anchors the strings. Consequently, when the ball is hit off center, the net effect of the decreased resilience is to: increase the shock, increase the turning or twisting forces as felt in the hand of the player. Even if the player can hold the racket firmly enough to resist the twisting forces, the ball will still move off the racket in unexpected and unwanted directions.
When the ball contacts the string, it will flatten. Therefore, the rate of change in resilience will affect the direction the ball leaves the striking face. For example, imagine the ball rising when it is struck. If the ball is struck near the top side of the string face with a flat stroke, the stiffer strings at the top side will tend to direct the ball downward into the net. If the same ball is struck with an intended top spin where the tendency would be to strike the ball near the bottom side of the hitting face, the stiffer strings at the bottom side will lift the ball higher than intended and direct the ball beyond the court.
Some tennis racket designers have made frame structural changes to make the string network a more uniform striking surface. The `Prince` tennis racket made the striking surface larger, proportionally wider and varied the string network spacing, closer near the center. The Wilson metal racket also made the striking head rounder and varied the spacing. C. L. Godfrey in his U.S. Pat. No. 1,733,960, Oct. 29, 1929, added additional string at the center. A new tennis racket market by Sabine Tennis Project under the trade name SweetSpot varied the spacing more than previously designed tennis rackets by eliminating both vertical and cross strings near the frame.